1. Field of the Invention
The present invention relates to a fuel cell system in which scavenging is performed during, for example, a halt of electricity generation by a fuel cell, and a method for operating the same.
2. Description of the Related Art
Recently, studies have been widely made to develop a fuel cell, such as polymer electrolyte fuel cell (PEFC), in which electricity is generated using hydrogen (fuel gas) supplied to an anode and oxygen-containing air (oxidant gas) supplied to a cathode. Such a fuel cell is formed by stacking a plurality of single cells, each formed by sandwiching a solid polymer electrolyte membrane between an anode and a cathode. When electricity is generated in the fuel cell, water is also generated at the cathode by an electrochemical reaction with hydrogen and oxygen.
When a fuel cell mounted on a vehicle or the like is used in a low-temperature environment (e.g., below zero), residual water (produced water) may be frozen and may damage a solid polymer electrolyte membrane and the like. Therefore, a process is required in which a scavenging gas is supplied to a cathode side to expel residual water (cathode scavenging), during a halt of electricity generation by the fuel cell. In addition, since water generated on the cathode side will penetrate through the solid polymer electrolyte membrane from the cathode to the anode, a process is required for the anode side to expel residual water (anode scavenging).
Accordingly, for example, JP2003-331893A (paragraphs 0022-0024 and FIGS. 1 and 3) discloses a technical idea in which, when a fuel cell system is used in a low-temperature environment, in order to prevent the produced water from being frozen in the fuel cell stack during a halt of electricity generation, cathode scavenging and anode scavenging are performed at the same time by switching valve positions of a pair of valves each communicating with either an anode inlet or a cathode inlet, to supply an unhumidified cathode gas (air) to both of the cathode and the anode in the fuel cell during the halt of electricity generation.
Meanwhile, when the fuel cell system disclosed in JP2003-331893A is used and, for example, cathode scavenging is performed first and anode scavenging is subsequently performed, most of a cathode gas (air) is supplied to the anode. Therefore, when the purge valve is opened and hydrogen purging is performed, there arises a problem that an amount of the cathode gas (air flow rate) to be introduced to the diluter provided downstream of the purge valve through the cathode is reduced, and that a concentration of hydrogen exhausted from the diluter is increased.
Accordingly, it would be desirable to provide a fuel cell system and an operation method therefor, in which high-concentration hydrogen is prevented from being exhausted to outside, even when hydrogen purging is performed during scavenging.
It would be also desirable to provide a fuel cell system and an operation method therefor, in which hydrogen purging during scavenging is smoothly performed to thereby shorten a time period for scavenging.